Please help me with this question.

See below in bold.

Its the second choice.

Step-by-step explanation:

The  sides b and d are the legs of a 30-60-90 triangle so the sides are in the ratio 2 : 1 :√3   where 2 = the hypotenuse.  

Here the hypotenuse = 10. Therefore:

10 = 2*5, so d = 1/2 * 10 and b = √3*5.

d = 5 , b =  5√3.

Now a , b and c are also the sides of a 30-60-90 triangle and we know that b = 5√3.

So  a ( the hypotenuse = 2 * 5√3 = 10√3.

and c ( the longest leg) =  5√3 *  √3 = 15.

you need a common difference between consecutive terms to be constant, like 1,2,3,4 or 2,4,6,8, or 1,3,6,9